1. Field of the Invention
The present invention relates to non-volatile memory devices and methods of driving same. More particularly, the invention relates to a non-volatile memory device that varies the level of a read voltage in response to variations in the distribution of a threshold voltage of a memory cell, as well as related methods of driving the non-volatile memory device.
2. Description of the Related Art
With the development of mobile consumer electronics and a variety of related applications, the demand for flash memory, one common form of non-volatile memory, is increasing. Flash memory is an electrically erasable and programmable non-volatile memory device that is able to retain stored data even when power is interrupted. A given quantity of data may be stored in flash memory using less power than that required to the same quantity of data in other conventional storage media, such as magnetic disc memory (i.e., hard disk drives or HDDs). Flash memory is also able to access stored data faster than HDD based memory systems.
Conventional flash memory is classified into NOR type and NAND type according to logical connection between the constituent memory cells and bit lines. NOR flash memory is formed by connecting at least two cell transistors to each bitline in parallel, and stores data using a channel hot electron method and erases data using a Fowler-Nordheim (F-N) tunneling method. NAND flash memory is formed by serially connecting at least two cell transistors to each bitline, and stores and erases data using an F-N tunneling method. In general, NOR flash memory is less capable of being densely integrated and consumes relatively more current, but also provides a relatively higher operating speed. On the other hand, NAND flash memory may be very densely integrated and consumes relatively less cell current than NOR flash memory.
Various multi level cell (MLC) architectures and related operating methods have been proposed to increase the overall data storage capacity of flash memory. Use of MLC allows a single memory cell to be programmed according to defined threshold voltages such the memory cell is able to store at least two bits of data. In contrast, flash memory using single level memory cells (SLC) store only one bit per memory cell.
FIG. 1A is a graph showing an exemplary threshold voltage distribution for a SLC. Since the SLC stores only one bit of data, the defined threshold voltages need only indicate two data states, [0] and [1]. In order to read the data value stored by memory cell of a SLC type flash memory, a single read voltage may be applied to a wordline associated with the memory cell. The read voltage has a value somewhere between the two threshold voltage distributions and is sufficient to discriminate the stored data value.
FIG. 1B is a graph showing an exemplary threshold voltage distribution for a MLC. More particularly, FIG. 1B illustrates a case where two bits of data are stored in a single memory cell. Since the MLC stores two-bit data, the defined threshold voltages must indicate four data states, [00], [01], [10], and [11]. It follows that in order to read data from a memory cell in a MLC type flash memory, three read voltages are needed.
In a SLC type flash memory, each memory cell may be programmed into one of two states using threshold voltage distributions that ensure a large discrimination (or read) margin between data states. In contrast, the read margins between respective data states for memory cells in a MLC type flash memory are greatly reduced. That is, respective adjacent threshold voltage distributions indicating the different data states are relatively close together.
Generally speaking, the threshold voltage of a flash memory cell varies in accordance with the quantity of electrical charge (i.e., electrons) stored on its floating gate. Unfortunately, electrons stored on a floating gate tend to dissipate or “leak” over time. In extreme cases, charge leakage may actual alter the threshold voltage of a programmed memory cell, thereby changing the stored data value. Due to the reduced read margins noted above, MLC memory systems are particularly susceptible to problems associated with charge leakage. The ability of a particular memory cell to retain charge over time is referred to as its retention characteristic. The retention characteristic of a memory cell is a reliability index of sorts.
FIG. 2 is a graph showing possible changes to the exemplary threshold voltage distributions of FIG. 1B as a result of charge leakage. As stored electrons dissipate from a floating gate with the passage of time, the threshold voltage distributions tend to shift or broaden. At some point, such shifting or broadening of the threshold voltage distributions substantially increase the likelihood of an erroneous data discrimination. For example, if a fixed read voltage, Vread[1], intended to distinguish the data values [10] and [01] is applied to the MLC of FIG. 2, the lower end of the broadened threshold voltage distribution for data [01] may fall below the read voltage Vread[1] due to charge leakage. Memory cells characterized by shifted or broadened threshold voltages are more likely to result in erroneous data interpretations. In conventional flash memory, a fixed read voltage is routinely used to discriminate stored data values even when the distribution of one or more threshold voltages for the memory cell is changed due to charge leakage. This approach is likely to result in errant data reads.